[PATCHv5 17/17] pps: add parallel port PPS signal generator

Alexander Gordeev
2010-11-24T16:25:09Z

Add PPS signal generator which utilizes STROBE pin of a parallel port to
send PPS signals. It uses parport abstraction layer and hrtimers to
precisely control the signal.

Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>
---
Documentation/pps/pps.txt | 46 +++++
drivers/pps/Kconfig | 2 +
drivers/pps/Makefile | 2 +-
drivers/pps/generators/Kconfig | 17 ++
drivers/pps/generators/Makefile | 9 +
drivers/pps/generators/pps_gen_parport.c | 275 ++++++++++++++++++++++++++++++
6 files changed, 350 insertions(+), 1 deletions(-)
create mode 100644 drivers/pps/generators/Kconfig
create mode 100644 drivers/pps/generators/Makefile
create mode 100644 drivers/pps/generators/pps_gen_parport.c

diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
index 125f4ab..d35dcdd 100644
--- a/Documentation/pps/pps.txt
+++ b/Documentation/pps/pps.txt
@@ -170,3 +170,49 @@ and the run ppstest as follow:

Please, note that to compile userland programs you need the file timepps.h
(see Documentation/pps/).
+
+
+Generators
+----------
+
+Sometimes one needs to be able not only to catch PPS signals but to produce
+them also. For example, running a distributed simulation, which requires
+computers' clock to be synchronized very tightly. One way to do this is to
+invent some complicated hardware solutions but it may be neither necessary
+nor affordable. The cheap way is to load a PPS generator on one of the
+computers (master) and PPS clients on others (slaves), and use very simple
+cables to deliver signals using parallel ports, for example.
+
+Parallel port cable pinout:
+pin name master slave
+1 STROBE *------ *
+2 D0 * | *
+3 D1 * | *
+4 D2 * | *
+5 D3 * | *
+6 D4 * | *
+7 D5 * | *
+8 D6 * | *
+9 D7 * | *
+10 ACK * ------*
+11 BUSY * *
+12 PE * *
+13 SEL * *
+14 AUTOFD * *
+15 ERROR * *
+16 INIT * *
+17 SELIN * *
+18-25 GND *-----------*
+
+Please note that parallel port interrupt occurs only on high->low transition,
+so it is used for PPS assert edge. PPS clear edge can be determined only
+using polling in the interrupt handler which actually can be done way more
+precisely because interrupt handling delays can be quite big and random. So
+current parport PPS generator implementation (pps_gen_parport module) is
+geared towards using the clear edge for time synchronization.
+
+Clear edge polling is done with disabled interrupts so it's better to select
+delay between assert and clear edge as small as possible to reduce system
+latencies. But if it is too small slave won't be able to capture clear edge
+transition. The default of 30us should be good enough in most situations.
+The delay can be selected using 'delay' pps_gen_parport module parameter.
diff --git a/drivers/pps/Kconfig b/drivers/pps/Kconfig
index 4823e47..79bda60 100644
--- a/drivers/pps/Kconfig
+++ b/drivers/pps/Kconfig
@@ -40,4 +40,6 @@ config NTP_PPS

source drivers/pps/clients/Kconfig

+source drivers/pps/generators/Kconfig
+
endmenu
diff --git a/drivers/pps/Makefile b/drivers/pps/Makefile
index 98960dd..1906eb70 100644
--- a/drivers/pps/Makefile
+++ b/drivers/pps/Makefile
@@ -4,6 +4,6 @@

pps_core-y := pps.o kapi.o sysfs.o
obj-$(CONFIG_PPS) := pps_core.o
-obj-y += clients/
+obj-y += clients/ generators/

ccflags-$(CONFIG_PPS_DEBUG) := -DDEBUG
diff --git a/drivers/pps/generators/Kconfig b/drivers/pps/generators/Kconfig
new file mode 100644
index 0000000..5fbd614
--- /dev/null
+++ b/drivers/pps/generators/Kconfig
@@ -0,0 +1,17 @@
+#
+# PPS generators configuration
+#
+
+if PPS
+
+comment "PPS generators support"
+
+config PPS_GENERATOR_PARPORT
+ tristate "Parallel port PPS signal generator"
+ depends on PARPORT != n && GENERIC_TIME
+ help
+ If you say yes here you get support for a PPS signal generator which
+ utilizes STROBE pin of a parallel port to send PPS signals. It uses
+ parport abstraction layer and hrtimers to precisely control the signal.
+
+endif
diff --git a/drivers/pps/generators/Makefile b/drivers/pps/generators/Makefile
new file mode 100644
index 0000000..303304a
--- /dev/null
+++ b/drivers/pps/generators/Makefile
@@ -0,0 +1,9 @@
+#
+# Makefile for PPS generators.
+#
+
+obj-$(CONFIG_PPS_GENERATOR_PARPORT) += pps_gen_parport.o
+
+ifeq ($(CONFIG_PPS_DEBUG),y)
+EXTRA_CFLAGS += -DDEBUG
+endif
diff --git a/drivers/pps/generators/pps_gen_parport.c b/drivers/pps/generators/pps_gen_parport.c
new file mode 100644
index 0000000..aa66803
--- /dev/null
+++ b/drivers/pps/generators/pps_gen_parport.c
@@ -0,0 +1,275 @@
+/*
+ * pps_gen_parport.c -- kernel parallel port PPS signal generator
+ *
+ *
+ * Copyright (C) 2009 Alexander Gordeev <lasaine@lvk.cs.msu.su>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+
+/*
+ * TODO:
+ * fix issues when realtime clock is adjusted in a leap
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/time.h>
+#include <linux/hrtimer.h>
+#include <linux/parport.h>
+
+#define DRVDESC "parallel port PPS signal generator"
+
+#define SIGNAL 0
+#define NO_SIGNAL PARPORT_CONTROL_STROBE
+
+/* module parameters */
+
+#define SEND_DELAY_MAX 100000
+
+static unsigned int send_delay = 30000;
+MODULE_PARM_DESC(delay,
+ "Delay between setting and dropping the signal (ns)");
+module_param_named(delay, send_delay, uint, 0);
+
+
+#define SAFETY_INTERVAL 3000 /* set the hrtimer earlier for safety (ns) */
+
+/* internal per port structure */
+struct pps_generator_pp {
+ struct pardevice *pardev; /* parport device */
+ struct hrtimer timer;
+ long port_write_time; /* calibrated port write time (ns) */
+};
+
+static struct pps_generator_pp device = {
+ .pardev = NULL,
+};
+
+static int attached;
+
+/* calibrated time between a hrtimer event and the reaction */
+static long hrtimer_error = SAFETY_INTERVAL;
+
+/* the kernel hrtimer event */
+static enum hrtimer_restart hrtimer_event(struct hrtimer *timer)
+{
+ struct timespec expire_time, ts1, ts2, ts3, dts;
+ struct pps_generator_pp *dev;
+ struct parport *port;
+ long lim, delta;
+ unsigned long flags;
+
+ /* NB: approx time with blocked interrupts =
+ send_delay + 3 * SAFETY_INTERVAL */
+ local_irq_save(flags);
+
+ /* first of all we get the time stamp... */
+ getnstimeofday(&ts1);
+ expire_time = ktime_to_timespec(timer->_expires);
+ dev = container_of(timer, struct pps_generator_pp, timer);
+ lim = NSEC_PER_SEC - send_delay - dev->port_write_time;
+
+ /* check if we are late */
+ if (expire_time.tv_sec != ts1.tv_sec || ts1.tv_nsec > lim) {
+ local_irq_restore(flags);
+ pr_err("we are late this time %ld.%09ld\n",
+ ts1.tv_sec, ts1.tv_nsec);
+ goto done;
+ }
+
+ /* busy loop until the time is right for an assert edge */
+ do {
+ getnstimeofday(&ts2);
+ } while (expire_time.tv_sec == ts2.tv_sec && ts2.tv_nsec < lim);
+
+ /* set the signal */
+ port = dev->pardev->port;
+ port->ops->write_control(port, SIGNAL);
+
+ /* busy loop until the time is right for a clear edge */
+ lim = NSEC_PER_SEC - dev->port_write_time;
+ do {
+ getnstimeofday(&ts2);
+ } while (expire_time.tv_sec == ts2.tv_sec && ts2.tv_nsec < lim);
+
+ /* unset the signal */
+ port->ops->write_control(port, NO_SIGNAL);
+
+ getnstimeofday(&ts3);
+
+ local_irq_restore(flags);
+
+ /* update calibrated port write time */
+ dts = timespec_sub(ts3, ts2);
+ dev->port_write_time =
+ (dev->port_write_time + timespec_to_ns(&dts)) >> 1;
+
+done:
+ /* update calibrated hrtimer error */
+ dts = timespec_sub(ts1, expire_time);
+ delta = timespec_to_ns(&dts);
+ /* If the new error value is bigger then the old, use the new
+ * value, if not then slowly move towards the new value. This
+ * way it should be safe in bad conditions and efficient in
+ * good conditions.
+ */
+ if (delta >= hrtimer_error)
+ hrtimer_error = delta;
+ else
+ hrtimer_error = (3 * hrtimer_error + delta) >> 2;
+
+ /* update the hrtimer expire time */
+ hrtimer_set_expires(timer,
+ ktime_set(expire_time.tv_sec + 1,
+ NSEC_PER_SEC - (send_delay +
+ dev->port_write_time + SAFETY_INTERVAL +
+ 2 * hrtimer_error)));
+
+ return HRTIMER_RESTART;
+}
+
+/* calibrate port write time */
+#define PORT_NTESTS_SHIFT 5
+static void calibrate_port(struct pps_generator_pp *dev)
+{
+ struct parport *port = dev->pardev->port;
+ int i;
+ long acc = 0;
+
+ for (i = 0; i < (1 << PORT_NTESTS_SHIFT); i++) {
+ struct timespec a, b;
+ unsigned long irq_flags;
+
+ local_irq_save(irq_flags);
+ getnstimeofday(&a);
+ port->ops->write_control(port, NO_SIGNAL);
+ getnstimeofday(&b);
+ local_irq_restore(irq_flags);
+
+ b = timespec_sub(b, a);
+ acc += timespec_to_ns(&b);
+ }
+
+ dev->port_write_time = acc >> PORT_NTESTS_SHIFT;
+ pr_info("port write takes %ldns\n", dev->port_write_time);
+}
+
+static inline ktime_t next_intr_time(struct pps_generator_pp *dev)
+{
+ struct timespec ts;
+
+ getnstimeofday(&ts);
+
+ return ktime_set(ts.tv_sec +
+ ((ts.tv_nsec > 990 * NSEC_PER_MSEC) ? 1 : 0),
+ NSEC_PER_SEC - (send_delay +
+ dev->port_write_time + 3 * SAFETY_INTERVAL));
+}
+
+static void parport_attach(struct parport *port)
+{
+ if (attached) {
+ /* we already have a port */
+ return;
+ }
+
+ device.pardev = parport_register_device(port, KBUILD_MODNAME,
+ NULL, NULL, NULL, 0, &device);
+ if (!device.pardev) {
+ pr_err("couldn't register with %s\n", port->name);
+ return;
+ }
+
+ if (parport_claim_or_block(device.pardev) < 0) {
+ pr_err("couldn't claim %s\n", port->name);
+ goto err_unregister_dev;
+ }
+
+ pr_info("attached to %s\n", port->name);
+ attached = 1;
+
+ calibrate_port(&device);
+
+ hrtimer_init(&device.timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
+ device.timer.function = hrtimer_event;
+#ifdef CONFIG_PREEMPT_RT
+ /* hrtimer interrupt will run in the interrupt context with this */
+ device.timer.irqsafe = 1;
+#endif
+
+ hrtimer_start(&device.timer, next_intr_time(&device), HRTIMER_MODE_ABS);
+
+ return;
+
+err_unregister_dev:
+ parport_unregister_device(device.pardev);
+}
+
+static void parport_detach(struct parport *port)
+{
+ if (port->cad != device.pardev)
+ return; /* not our port */
+
+ hrtimer_cancel(&device.timer);
+ parport_release(device.pardev);
+ parport_unregister_device(device.pardev);
+}
+
+static struct parport_driver pps_gen_parport_driver = {
+ .name = KBUILD_MODNAME,
+ .attach = parport_attach,
+ .detach = parport_detach,
+};
+
+/* module staff */
+
+static int __init pps_gen_parport_init(void)
+{
+ int ret;
+
+ pr_info(DRVDESC "\n");
+
+ if (send_delay > SEND_DELAY_MAX) {
+ pr_err("delay value should be not greater"
+ " then %d\n", SEND_DELAY_MAX);
+ return -EINVAL;
+ }
+
+ ret = parport_register_driver(&pps_gen_parport_driver);
+ if (ret) {
+ pr_err("unable to register with parport\n");
+ return ret;
+ }
+
+ return 0;
+}
+
+static void __exit pps_gen_parport_exit(void)
+{
+ parport_unregister_driver(&pps_gen_parport_driver);
+ pr_info("hrtimer avg error is %ldns\n", hrtimer_error);
+}
+
+module_init(pps_gen_parport_init);
+module_exit(pps_gen_parport_exit);
+
+MODULE_AUTHOR("Alexander Gordeev <lasaine@lvk.cs.msu.su>");
+MODULE_DESCRIPTION(DRVDESC);
+MODULE_LICENSE("GPL");
--
1.7.2.3